At a NIH Consensus Development Meeting on Steroid Receptors in Breast Cancer in 1979 it was concluded: "--new techniques that localize receptors within tumor cells are indeed desirable and may provide new or additional information. At the present, however, no histochemical receptor assay can be validated and further work is needed to establish whether any such method has clinical usefulness--". This situation exists today. The most promising histochemical technique appears to be immunocytochemistry with monoclonal antibodies to estrogen receptor. To date, however, all antibodies have been raised against "purified" receptors. The problem has been that other, non-receptor proteins, co-purify with the receptor and their are questions as to whether the antibody is really made against the receptor itself. This proposal is designed to develop a monoclonal antibody to estrogen receptor without relying on the purification of receptor. An anti-idiotypic antibody will be made to estradiol by first making a monoclonal antibody (A) to estradiol and then by making a second antibody, the anti-idiotypic antibody, to A,. Of the anti-idiotypic antibodies to estradiol, several will be selected for which also bind to estrogen receptor. They will then be characterized and a procedure will be developed for immunocytochemical localization of estrogen receptor. Finally, the localization found with the anti-idiotypic antibodies will be compared with that found with a antibodies made against purified receptor and with that found with autoradiography.